1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, a pouch type secondary battery with a safety vent, which can prevent explosion and fire.
2. Description of the Related Art
Secondary batteries are generally rechargeable and can be made into a smaller size with high capacity. Typical examples of secondary batteries include nickel-metal hydride (Ni-MH) batteries and lithium (Li) secondary batteries.
Secondary batteries are classified into cylindrical batteries using a cylindrical aluminum can, rectangular batteries using a rectangular aluminum can, and pouch type batteries accommodated in a thin-film pouch type casing, according to the outer appearance of a casing.
Specifically, a pouch type battery is fabricated by encasing an electrode assembly which generates current in a pouch casing made of a flexible film material, and sealing the same. When the internal pressure of such a pouch type battery increases due to overcharge, gas is released, which may cause swelling and explosion of a battery pack. In particular, when a lithium ion battery is overcharged, decomposition of a liquid electrolyte leads to a release of gas, such as carbon dioxide or carbon monoxide, increasing the internal pressure of the battery. Also, if over-current flows through a battery due to overdischarging or short-circuit, the internal pressure and temperature of the battery increase, leading to a risk of fire, which is a serious problem from the safety point of view.
In order to ensure safety, such secondary batteries must meet temperature requirements for charge and use. Under severe use conditions, that is, when the temperature during charging/discharging or the operation temperature of a device using batteries is excessively high, the internal temperature of a battery rises, resulting in explosion and fire.
Thus, in view of safety, manufactured secondary batteries are subjected to various safety tests, including tests of safety against overcharge or forced discharge, and thermal safety tests such as high temperature storage testing, thermal shock testing or thermal exposure testing, before commercial distribution. The thermal safety tests are carried out by allowing batteries to stand under various temperature conditions for a plurality of minutes to a plurality of hours. Even under such conditions, explosion and fire do not occur in batteries meeting safety requirements. However, in extreme cases, a battery seal is removed, thus preventing explosion and fire.
Various attempts have been made to overcome such a safety problem in secondary batteries. In particular, techniques have been vigorously sought to remove internal gases of a battery by installing a safety vent on a battery pack. By way of example, Korean Patent Publication No. 98-44210 discloses a secondary battery configured to provide a separate gas outlet or check valve made of a material having a low burst resistance at a front upper end of the battery. The gas outlet is designed to be ruptured or to exhaust internal gas through the check valve when the battery is swollen due to overcharging or overdischarging. However, the necessity of providing a separate device at the upper end of the battery requires an additional process in the manufacture of batteries, deteriorating manufacturability.
U.S. Pat. No. 4,678,725 discloses that a thermally fusible portion of an electrode tap provided at the upper end of a battery ruptures, when the internal pressure rises to a predetermined level of pressure, to allow internal gas to be exhausted outside the battery. However, according to this technique, installing a separate safety vent requires modification of a battery pack, and it is necessary to manufacture a separate casing that is different from a conventional casing. Even though the conventional casing is used, since the safety vent has a unsealed portion, and a clip is placed on the unsealed portion, the burst strength of the clip must be taken into consideration for adhesion, increasing the number of processing steps and deteriorating breaking reliability.
Japanese Patent Laid-open Publication No. 2000-100399 discloses an outer film container of a polymer lithium secondary battery, wherein the outer container casing comprises thermally fusible layers adhered to each other to hermetically seal a space in which an electrode assembly is housed, and wherein a portion of the outer container casing is a breaking portion that fuses at a lower temperature than a temperature at which a non-breaking portion fuses, that is, the fusibility of the breaking portion is lowered. Accordingly, when the internal pressure of the battery is elevated to a predetermined value, the breaking portion is ruptured.
However, it is quite difficult to reduce only the fusing temperature of the breaking portion. Also, adjusting a fusible bonding force at the breaking portion is quite difficult to achieve, which is disadvantageous in view of the manufacturability and the reliability of a safety vent.